Methane (CH4) is produced in many natural systems that are vulnerable to change under a warming climate, yet current CH4 budgets, as well as future shifts in CH4 emissions, have high uncertainties. Climate change has the potential to increase CH4 emissions from critical systems such as wetlands, marine and freshwater systems, permafrost, and methane hydrates, through shifts in temperature, hydrology, vegetation, landscape disturbance, and sea level rise. Increased CH4 emissions from these systems would in turn induce further climate change, resulting in a positive climate feedback. Here we synthesize biological, geochemical, and physically focused CH4 climate feedback literature, bringing together the key findings of these disciplines. We discuss environment‐specific feedback processes, including the microbial, physical, and geochemical interlinkages and the timescales on which they operate, and present the current state of knowledge of CH4 climate feedbacks in the immediate and distant future. The important linkages between microbial activity and climate warming are discussed with the aim to better constrain the sensitivity of the CH4 cycle to future climate predictions. We determine that wetlands will form the majority of the CH4 climate feedback up to 2100. Beyond this timescale, CH4 emissions from marine and freshwater systems and permafrost environments could become more important. Significant CH4 emissions to the atmosphere from the dissociation of methane hydrates are not expected in the near future. Our key findings highlight the importance of quantifying whether CH4 consumption can counterbalance CH4 production under future climate scenarios.

Partial-nitritation anammox (PNA) is a novel wastewater treatment procedure for energy-efficient ammonium removal. Here we use genome-resolved metagenomics to build a genome-based ecological model of the microbial community in a full-scale PNA reactor. Sludge from the bioreactor examined here is used to seed reactors in wastewater treatment plants around the world; however, the role of most of its microbial community in ammonium removal remains unknown. Our analysis yielded 23 near-complete draft genomes that together represent the majority of the microbial community. We assign these genomes to distinct anaerobic and aerobic microbial communities. In the aerobic community, nitrifying organisms and heterotrophs predominate. In the anaerobic community, widespread potential for partial denitrification suggests a nitrite loop increases treatment efficiency. Of our genomes, 19 have no previously cultivated or sequenced close relatives and six belong to bacterial phyla without any cultivated members, including the most complete Omnitrophica (formerly OP3) genome to date.

Arctic permafrost soils store large amounts of organic matter that is sensitive to temperature increases and subsequent microbial degradation to methane (CH ) and carbon dioxide (CO ). Here, we studied methanogenic and methanotrophic activity and community composition in thermokarst lake sediments from Utqiag˙vik (formerly Barrow), Alaska. This experiment was carried out under in situ temperature conditions (4°C) and the IPCC 2013 Arctic climate change scenario (10°C) after addition of methanogenic and methanotrophic substrates for nearly a year. Trimethylamine (TMA) amendment with warming showed highest maximum CH production rates, being 30% higher at 10°C than at 4°C. Maximum methanotrophic rates increased by up to 57% at 10°C compared to 4°C. 16S rRNA gene sequencing indicated high relative abundance of Methanosarcinaceae in TMA amended incubations, and for methanotrophic incubations Methylococcaeae were highly enriched. Anaerobic methanotrophic activity with nitrite or nitrate as electron acceptor was not detected. This study indicates that the methane cycling microbial community can adapt to temperature increases and that their activity is highly dependent on substrate availability.

Permafrost covers a quarter of the northern hemisphere land surface and contains twice the amount of carbon that is currently present in the atmosphere. Future climate change is expected to reduce its near-surface cover by over 90% by the end of the 21st century, leading to thermokarst lake formation. Thermokarst lakes are point sources of carbon dioxide and methane which release longterm carbon stocks into the atmosphere, thereby initiating a positive climate feedback potentially contributing up to a 0.39°C rise of surface air temperatures by 2300. This review describes the potential role of thermokarst lakes in a warming world and the microbial mechanisms that underlie their contributions to the global greenhouse gas budget. The Warming Arctic Faces Permafrost Degradation and Thermokarst FormationPermafrost covers a quarter of the northern hemisphere land surface [1,2]. Its carbon (C) pools are estimated at 1300 Pg C (range 1100-1500 Pg C) which equals twice the amount of carbon that is present in the atmosphere [3,4]. The major fraction, about 1000 Pg, is present in the near surface (the upper 3 m) that is vulnerable to warming [3]. Warming leads to destabilization and degradation of permafrost landscapes [5]. The Arctic Climate Impact Assessment (ACIA) reported a two times higher rise in Arctic air surface temperatures compared with the global average [6,7], resulting in an average permafrost soil and sediment warming of 0.29 ± 0.12°C between 2007 and 2016 [8]. This disproportionate near-surface warming is known as the 'Arctic amplification' [9]. It has a pronounced effect on near-surface permafrost which is expected to be reduced by over 90% at the end of the 21st century [10]. Upon thaw, the increased biological availability of carbon leads to enhanced microbial greenhouse gas (GHG) production [11][12][13][14] and net GHG emissions which consist mainly of CO 2 and CH 4 [15][16][17].One consequence of a warming Arctic and permafrost thaw is the formation of thermokarst (see Glossary) landscapes. These landscapes are associated with pingos, thermokarst troughs and pits that can collapse to form thermokarst lakes [18]. GHG emissions from these lakes can have disproportionate climate effects due to the rapid release of long-term stored carbon into the atmosphere, which initiates a strong positive climate feedback [17,[19][20][21][22]. This review focuses on the role of thermokarst lakes in a warming world and the microbial mechanisms that underlie their contributions to the global GHG budget.Thermokarst Lakes Are a Feature of Thawing Yedoma Deposits About 40% (500 Pg) of the permafrost carbon stocks are found in ice-rich Yedoma deposits which have an organic carbon content ranging from 2% to 5% [23,24]. These deposits are found in Alaska and Siberia, and they originate from the late Pleistocene [25][26][27]. The ice-rich Yedoma deposits remained unglaciated during the last ice age and contain organic carbon deposits from alluvial plains, hillslopes, and polygonal lowlands [28]. Yedoma landscapes are highly sens...